In the field of particle physics, various situations call for measuring time differences at extremely high resolutions, for example, in order to determine the velocities of elementary particles with a high degree of precision. For example, in what are generally referred to as time-of-flight (TOF) experiments, the period of time between the formation of a particle and its impinging upon a fast time-resolving detector, is determined. In this connection, what is commonly known as a start signal is generated, for example, by another time-resolving detector. The first mentioned detector likewise produces a signal that is used as a stop signal. A time-measuring instrument measures the time difference between the two signals, and, for example, the velocity of the particle can be measured based on the time difference, partly aided by other measurement data. Experts in the field refer to such a measurement as a ‘coincidence measurement.’ Frequently, the time-measuring instrument is also started by a detector signal and stopped by a machine clock, for example, in the case of a particle accelerator operated in pulsed mode. In this case, it is not problematic that the inverse time-of-flight is measured.
The detector signals typically undergo a plurality of processing steps in order to obtain a precise signal timing. An example of such a processing step is the use of amplifiers and discriminators. The latter output a standard signal that is in a precise temporal relationship with the corresponding detector signal. Such measuring methods, coincidence measurements and signal processing processes are reasonably well known to one skilled in the art.
Time-to-amplitude converters (abbreviated TAC) or time-to-digital converters (abbreviated TDC) are typically used as time-measuring instruments. Both measure the time difference between the start signal and the stop signal which are fed into the device in question.
Thus, a TAC typically has a start input and a stop input which each receive a transmitted standard signal. In the case of a TAC, the time difference between receipt of the start signal and the stop signal is converted into an analog voltage signal, whose signal height is typically proportional to the time difference. This analog signal can then either be further processed in analog form or, for example, be digitized by a downstream analog-digital converter (ADC). On the other hand, in the case of a TDC, the time difference is directly digitized, for example, in order to be instantaneously read out by a computer and further processed; i.e., in the case of a TDC, analog signals are not used in the process.
An example of the application of time-to-amplitude converters is particle identification using the planned CBM (Compressed Baryonic Matter) detector to be implemented in the course of the FAIR (Facility for Antiproton and Ion Research) project of the applicant. Depending on the experiment, the time-of-flight can be used to obtain information about the particles, for example, the mass, mass/charge ratio, velocity, etc., thereof. These experiments mostly require an extremely high-resolution time measurement at an accuracy well below the nanosecond region.
A time-to-amplitude converter already previously developed by the applicant is described in the German Patent DE 195 33 414 C1. FIG. 1 shows a block diagram of this type of converter, which is based on a delay chain of CMOS inverter pairs that charge a capacitor via a resistor network. Upon receipt of a start signal, a signal is transmitted through the delay chain. The capacitor is charged via a parallel array of tristate drivers and the resistor network in such a way that the capacitor voltage increases linearly over time. The tristate drivers described, for example, in N. H. E. Weste, K. Eshraghian: Principles of CMOS VLSI Design, Addison-Wesley, 1994 (page 91) or R. J. Baker, H. W. Li, D. E. Boyce: CMOS Circuit Design, Layout, and Simulation (page 226), to which reference is hereby made and whose content is incorporated by reference herein. Upon receipt of the stop signal, the tristate drivers separate the resistor network from the delay chain, and the ramp signal remains at a fixed voltage value U that is proportional to the time interval between receipt of the start signal and the stop signal. Once analog value U has been read out, the converter can be reset by a reset signal at a reset input.
For further details pertaining to such a time-to-amplitude converter, reference is made to the German Patent DE 195 33 414 C1, which is incorporated by reference herein.
H. Correia et. al: Delay, A 4 Channel ½ ns Programmable Delay Line Chip Manual, version 1.1 CERN, May 2005 describes coupling a delay chain as such to a DLL. However, this chip is merely used for a defined signal delay.
In principle, TAC components have been tried and tested. However, it turns out that, in view of the extreme requirements that the inventors place on accuracy and reproducibility, there is still a need for further improvement. In particular, it has become evident that the delay time of the delay elements is subject to temperature-dependent fluctuations. Moreover, it has become evident that the components are also subject to process- or charge-dependent tolerances. This means that, as long as two TAC components are produced on the same wafer, the fluctuations are relatively minor. However, if two TAC components from different wafers are compared, they disadvantageously have different delay times, which leads to a degradation of the time resolution of the converter if adequate countermeasures are not taken.
For that reason, methods have provided for recording calibration data in order to allow for these dependencies; i.e., the converters have always been at least calibrated to the prevailing ambient temperature. This process does, in fact, work, but it is involved and requires a constant temperature over the time period of the measurement.